1. Field of the Invention
The present invention relates to optical collimating devices and methods for making them, and more particularly to collimating devices that use epoxy and a tube to join components thereof.
2. Description of the Prior Art
Demand for higher transmission capacity and speed in optical communications systems is unrelenting. Optical transmission technology is constantly developing to satisfy such demand. Dense Wavelength Division Multiplexing (DWDM) technology has been an important development, and is now in widespread use in optical communications systems throughout the world. A DWDM system multiplexes a plurality of signals of different wavelengths into a single optical fiber at an initiating end of the optical fiber. The multiplexed signals are then demultiplexed into a plurality of signals at a terminal end of the optical fiber. Each demultiplexed signal is then output to an end recipient. DWDM systems can increase optical transmission capacity by up to ten times or even more.
A collimating device incorporating a filter is a basic wave division device deployed in many optical modules used in DWDM systems, such as Optical Add-Drop Multiplexers (OADMs) and Dense Wavelength Division Multiplexers (DWDMs). FIG. 1 shows a conventional collimating device 10 comprising a Graded Index (GRIN) lens 12 and a filter 14. Typically, the filter 14 is adhered on an end face (not labeled) of the GRIN lens 12 using two types of epoxy. One type is UV Epoxy 16, and the other type is 353 NDT Epoxy 18. UV Epoxy 16 has less fluidity and viscosity than 353 NDT Epoxy 18. In assembly, UV Epoxy 16 is first applied at an interface (not labeled) between the GRIN lens 12 and the filter 14. A UV gun temporarily cures the UV Epoxy 16. This process sometimes results in contamination of the corresponding end faces of the GRIN lens 12 and the filter 14. Such contamination reduces optical performance of the collimating device 10. Then the 353 NDT Epoxy 18 is applied around an outer surface of the UV Epoxy 16. A relatively large amount of 353 NTD Epoxy 18 is required to cover the entire expanse of the interface. The collimating device 10 is baked to permanently cure the 353 NTD Epoxy 18 and the UV Epoxy 16. The GRIN lens 12 and filter 14 are thus fastened to each other.
Coefficients of thermal expansion of UV Epoxy 16 and 353 NTD Epoxy 18 are respectively 4.3xc3x9710xe2x88x925/xc2x0 C. and 4.7xc3x9710xe2x88x925/xc2x0 C., both of which are several times greater than coefficients of thermal expansion of the GRIN lens 12 and the filter 14. Therefore, baking can result in asymmetric thermal stress. As a result, optical characteristics of the collimating device 10 may vary according to the prevailing operating temperature of the collimating device 10 at any given time. Unstable operation of the collimating device 10 adversely affects signal transmission. Moreover, the procedure for obtaining precise alignment and positioning of the filter 14 relative to the GRIN lens 12 is complicated. This increases production costs.
An improved collimator device is needed to overcome the many disadvantages of conventional collimating devices.
Accordingly, an object of the present invention is to provide an inexpensive collimating device having low insertion loss.
Another object of the present invention is to provide a collimating device free from contamination caused by excess glue.
A further object of the present invention is to provide a collimating device which allows easy and precise adjustment of components thereof.
To achieve the objects set out above, a collimating device of the present invention comprises a GRIN lens, a filter and an outer tube. The GRIN lens and the filter are both secured in the outer tube. The outer tube is made of stainless steel, and has a first receiving portion and a second receiving portion. The first receiving portion is cylindrical, and defines a cylindrical cavity therein for receiving the GRIN lens. The second receiving portion is also cylindrical, and defines a generally rectangular cavity therein for receiving the filter. The GRIN lens and the filter are glued in the outer tube with relatively little epoxy, and with no excess epoxy contaminating optical faces thereof.
Other objects, advantages and novel features of the present invention will be apparent from the following detailed description of a preferred embodiment thereof with reference to the attached drawings, in which: